Although astronomers who work with ground-based telescopes have to deal with bad weather and atmospheric filtering, they do have one advantage over astronomers working with instruments in space. The ground-based astronomers can work directly with their instruments. That means that they can constantly check and adjust their instruments first-hand. Astronomers working with satellite-based instruments must do everything remotely. With the exception of telescopes mounted in the Space Shuttle's payload bay and the Hubble Space Telescope, which was serviced by Shuttle astronauts in 1993, astronomers can only interact with their instruments via radio transmissions. That means that the instruments have to be mounted on a satellite that provides radio receivers and transmitters, electric power, pointing control, data storage, and a variety of computer-run subsystems.

Data collection, transmission, and analysis is of primary importance to astronomers. The development of photomultiplier tubes and CCDs or charged coupled devices (See introduction in Unit 3.) provides astronomers with an efficient means of collecting data in a digital form, transmitting it via radio, and analyzing it by computer processing. CCDs, for example, convert photons falling on their light sensitive elements into electric signals which are assigned numeric values

representing their strength. Spacecraft subsystems convert numeric values into a data stream of binary numbers that are transmitted to Earth. Once received, computers reconvert the data stream to the original numbers that can be processed into images or spectra.

If the satellite is in a geostationary orbit, which permits it to remain above one location on Earth, these data may be continuously transmitted to ground receiving stations consisting of one or more radio antennas and support equipment. Geostationary satellites orbit in an easterly direction over Earth's equator at an elevation of approximately 40,000 kilometers. They orbit Earth in one day, the same time it takes Earth to rotate, so the satellite remains over the same part of Earth at all times.

Satellites at other altitudes and orbital paths do not stay above one point on Earth. As a result, they remain visible to a particular ground station for a short time and then move out of range. This requires many widely-spaced ground stations to collect the satellite's data. In spite of this, the satellite still spends much of its time over parts of Earth where no stations exist (oceans, polar regions, etc.). For this reason, one of the subsystems on astronomical satellites are tape recorders that store data until they can transmit it to ground stations.

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